The assembly of some metal structures such as airplane wings includes fastening several components of the structure together with rivets or similar fasteners. Prior to assembly, a multitude of fastener holes are formed in the components. As is well known in the art, holes or other discontinuities in a structure lead to localized stress concentrations near the hole when the structure is loaded. These stress concentrations can lead to early fatigue failure of the structure. The fatigue life of the structure can be greatly improved if the material surrounding a fastener hole is strain hardened through a process known as cold-working. Cold-working is the process of plastically deforming the material around the hole at a temperature below its annealing point in order to increase is hardness and tensile strength.
Cold-working the metal that surrounds fastener holes can be performed in a variety of ways. One method involves lining the hole with a sleeve and pulling a shaft having an enlarged diameter portion (commonly referred to as a mandrel) through the hole to radially expand the hole and achieve the desired strain hardening of the metal. Specifically, a longitudinally split annular metal sleeve is slid onto the mandrel over its enlarged diameter portion. The enlarged diameter portion of the mandrel, which is slightly smaller than the hole, is then passed through the hole. Next, the sleeve is slid along the mandrel into position within the hole. The sleeve has an inside diameter that is smaller than the enlarged diameter portion of the mandrel. With the sleeve held in place within the hole, the mandrel is then forcibly withdrawn through the hole by a suitable pulling mechanism so that the enlarged diameter portion of the mandrel forces radial expansion of the sleeve and hole as it passes through. The desired strain hardening is created by the forced expansion of the hole compressing the region surrounding the periphery of the hole. The sleeve is then removed from the hole and discarded before the fastener is installed. Champoux, U.S. Pat. No. 4,187,708, discloses an apparatus and method for performing this type of cold-working.
At least three problmes arise when sleeves are employed in a cold-working process. Firstly, the metal sleeves, which are typically formed of stainless steel, can only be used once and are therefore costly to supply in large numbers. Secondly, since the sleeves must be split to fit over the enlarged diameter portion of the mandrel, the forced radial expansion of the split sleeve within the hole results in a protruding ridge formed in the hole that must be removed by reaming before a fastener can be inserted. Thirdly, the sleeve itself undergoes some compression during the cold-working process. Thus, not all of the compressive force developed by pulling the mandrel through the hole is applied to compressing (i.e., strain hardening) the material surrounding the hole.
One alternative to cold-working with sleeves includes inserting a multipart collapsible mandrel in collapsed form through a hole, and mechanically expanding the mandrel. The expanded mandrel is then pulled back through the hole to compress the region surrounding it. Two disadvantages of this technique are that collapsible mandrels are expensive to build and do not have the strength of solid mandrels. Furthermore, when expanded, the gaps between the parts of a multipart mandrel leave ridges in the periphery of a hole as the mandrel is drawn through the hole. These ridges must be removed by reaming before a fastener can be inserted. When reaming exposed ridges, it is practically impossible to avoid simultaneous removal of a portion of the hole's sidewall. It is this portion of the metal that is closest to the hole that receives the greatest amount of compression (hence, produces the greatest fatigue resistance) and preferably should not be removed.
In order to avoid the use of sleeves or collapsible mandrels with their attendant problems, solid mandrels having enlarged diameter portions that are larger than the hole have been employed in the past. The solid mandrels are unattached to any push or pull mechanisms and are forced completely through the hole with suitable driving means such as a pneumatic hammer. However, such a process requires substantial clearance and access on each side of the hole in order to retrieve the mandrel. Such access is difficult to obtain in many airplane wing portions. Furthermore, even if access is available, retrieval of the mandrel each time a hole is cold-worked increases labor costs.
Regardless of whether sleeved or multipart mandrels are employed in the cold-working process, the mandrel will typically leave an annular protrusion of metal around the edge of the hole on the side of the workpiece from which the mandrel exits the hole. Such a protrusion is also found when a solid mandrel is forced completely through the hole in one direction. In most cases, this protrusion must be removed before the fastener is inserted.